Дети сперва играли порознь, а потом вместе начали строить башню из кубиков.

Дети means children and it is the nominative plural form, used because children are the subject of the sentence.

A useful thing to know is that ребёнок (child) has an irregular plural:

• singular: ребёнок
• plural: дети

So you cannot just make the plural in a regular way.

Сперва means at first or first.

In this sentence, сперва and потом create a clear sequence:

• сперва = at first
• потом = then / afterward

Very often, сперва and сначала are close in meaning and can both mean at first. In many everyday contexts, they are interchangeable. Сперва can sound a little more conversational or stylistically lighter, but both are common.

Играли is the past tense plural of играть, which is an imperfective verb.

It is used here because the sentence describes an ongoing activity in the past:

• Дети сперва играли порознь = The children were playing separately at first

The focus is on the process or activity, not on completing it.

A form like сыграли would usually suggest a completed action, often something more like played through / played a game / finished playing, which does not fit as naturally here.

Порознь means separately, apart, or each on their own.

So:

• играли порознь = played separately

It is an adverb. It describes how they were playing.

A learner might also compare it with:

• отдельно = separately
• по отдельности = individually / separately

All of these can be similar, but порознь has a natural sense of not together.

Yes, very normal. The sentence contrasts two stages:

• сперва играли порознь = at first they played separately
• а потом вместе начали строить... = but then together they started building...

So порознь and вместе are deliberately contrasted:

• порознь = separately
• вместе = together

This makes the change in the children’s behavior very clear.

Here а marks a contrast or shift:

• at first ... separately
• but then ... together

So а потом is very natural because the second part contrasts with the first.

Compare:

• и = and, simple addition
• а = and/but, with contrast or change of situation

In this sentence, а is better because the children go from being separate to acting together.

This is a very common Russian structure:

• начать + infinitive = to begin/start doing something

So:

• начали строить = started to build

Here:

• начали = began / started (past plural)
• строить = to build

Russian often uses this structure just like English uses started + infinitive or started + -ing.

This is a very common and important pattern.

• начали comes from начать, a perfective verb, because starting is treated as a completed event: they did start.
• строить is imperfective, because after the start, the building is viewed as a process.

So the sentence means:

• they completed the action of beginning
• and the action they began was the ongoing activity of building

This combination is extremely natural in Russian:

• начал читать = started reading
• начала писать = started writing
• начали строить = started building

Because башню is the accusative singular form of башня.

The verb строить usually takes a direct object:

• строить что? = build what?
• башню = a tower

Since башня is a feminine noun, its accusative singular changes:

• nominative: башня
• accusative: башню

So:

• строить башню = to build a tower

Из кубиков means out of blocks or from blocks.

After из, Russian normally uses the genitive case, so:

• кубики = nominative plural
• кубиков = genitive plural

This is a standard pattern:

• из дерева = out of wood
• из бумаги = out of paper
• из кубиков = out of blocks

So башню из кубиков literally means a tower out of blocks.

In this context, кубики usually means toy blocks, especially the kind children build with.

The basic singular is:

• кубик = little cube / block

In a sentence about children building a tower, an English speaker should usually understand из кубиков as from toy blocks, not as abstract geometric cubes.

Yes, Russian word order is flexible, but the given order is very natural.

This sentence is arranged to show time and contrast clearly:

• Дети — the topic/subject
• сперва — first stage
• играли порознь — what they were doing
• а потом — transition to the next stage
• вместе — contrast with порознь
• начали строить башню из кубиков — new joint action

You could change the word order, but it would affect emphasis. For example:

• Сперва дети играли порознь... also sounds natural

The original sentence is smooth and easy to follow.

Russian has no articles like a or the.

So:

• дети can mean children or the children
• башню can mean a tower or the tower

The exact meaning depends on context. In this sentence, English naturally uses:

• The children
• a tower

But Russian does not need separate article words to express that.

Because а is a coordinating conjunction joining two clauses, and in Russian this normally requires a comma.

The two parts are:

• Дети сперва играли порознь
• а потом вместе начали строить башню из кубиков

So the comma marks the division between the first situation and the contrasting second one.

Yes. Russian allows some movement of adverbs like вместе, depending on emphasis.

For example, these are possible:

• ...а потом вместе начали строить башню...
• ...а потом начали вместе строить башню...

The original version puts вместе early, which strongly highlights the contrast with порознь. That is probably why it is placed there.